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Post by Deleted on Nov 24, 2015 7:51:58 GMT
I know a lot has been written about modifying the backstay to make it adjustable. I want to make the modification according to what was done on a 36i. (see backstay modification ) The forces of the top and middle block are mentioned within the drawing but that of the triplex blocks is not mentioned. Can anybody advise on what force to take into account when selecting the right blocks.
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Post by rene460 on Nov 25, 2015 8:58:36 GMT
Hi marien,
I would work it out this way- in addition to the two top blocks, the link also mentions the backstay as 6 mm 1x19 SS. The Selden guide gives the breaking strength as 31kn, roughly 3000 kg, so consistant with the top block. Because of the V, the two legs would each be a bit above half, but 1400 kg for the next block is only slightly less, I would guess due to available pulley load ratings, but roughly equal strength. At 3000 kg total the top triple would see approx 750 kgspread over 6 parts of the rope, while the lower one would see 7/6 of this depending on the exact angle at which the adjusment pull is applied and ignoring friction in all the estimates. So to answer your question, the two triple blocks would need to be about 700 to 800 kg. to make all components about equal strength. Rope pull to achieve this would be about 120 kg!! It is not predictable which of the nearly equal components would break first if the system is ever overloaded to breaking point.
However there are two other issues implied by your question. The Selden guide advises shrouds to be tensioned only to 15-20% of breaking strength, if the criterion is applied to the back stay, say a max of 600 kg which would require a max adjustment force of around 25 kg in round figures, so definitely not a winch job after all, and a reasonable guide to the max tension you should apply.
The other question in my mind is why you are considering this change? An adjustable back stay in my understanding goes with a bendy mast, and is used to bend the mast to flatten the main. Pure racing boats often have a very flexible mast as part of this system. I have not checked what you have but most of our boats have something more like a telephone line pole, intended to be nearly straight, not really flexible, though a small bends something much less than the width of the mast is possible and not uncommon. Backstay tension helps tension the forestay but the shrouds are also helpful for this, especially when attached to the mast close to the height of the forestay attachment, and excessive backstay tension can start bending the hull and will certainly affect your motor to shaft alignment. Hence I am not sure why you need more than a normal turnbuckle. If the backstay is a bit long, and you have a split backstay, a turnbuckle each side gives you a bit more range. Some very experienced members have mentioned adjusting rig tension for expected wind conditions before a race, but I do not believe it is normally considered a sail trim adjustment through the race for most of us.
I hope this this gives you a starting point and that a few more more members will add more information.
rene460
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Post by Deleted on Nov 25, 2015 18:35:36 GMT
Thanks Rene for your answer. I was wondering what is mend with the block force. Is this the force each wheel can handle or is it the total breaking strength of the block housing. So as I understand with this tripple block each rope length takes the force of 1/3 of the block which results in a total strength with a 700kg block of 2100kg.
I am not really sure about your point of view whether or not to modify the backstay. I have heard from so many people that it is a real help in trimming the sails. My first boat was a Dufour 2800 en I used it really a lot. The Sun Fast 37 like all sporty boats has an adjustable backstay. Since the SO37 has an equal hull I cannot imagine it would bend due to the tension of the backstay.
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Post by sailby2 on Nov 26, 2015 0:44:33 GMT
Marien, I fitted an adjustable backstay to my 2001 SO37. I retained the existing backstay from the masthead down to the split and attached a wire block. A rigger made a bridle of wire running from starboard to about 1.5m above the port side attachment. We used a multi purchase system shown in the attached picture. What I really appreciated was being able to take out most of the headstay sag thus enabling me to point higher. Was it worth it? I think so and really miss not having a similar set up on my current SO42i. I used Harken Carbo blocks for the smaller multi purchase assembly and never felt like it was at risk. I have seen where some racing rigs use a Dyneema or other high strength line as a safety line between the wire bridle thimble and the on deck attachment point. By the time we sold the boat, we had around 4000nm on the rig with this set up. Regards,
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Post by rene460 on Nov 26, 2015 12:30:55 GMT
Hi marien,
I believe the breaking load for the block is the total load at the hanger shackle, which is the same as the total load of all the ropes pulling the other directions. Which component of the block is the weakest link will vary between different block designs and manufacturers. It could be the cheeks or the shaft or the sheave, the shackle pin or other component. A triple is rarely three times as strong as a single.
For a single block there are two ropes leaving the sheave, each takes half the load. Or if you prefer it, one rope over the sheave, each end of which has equal tension and takes half the total load on the block. A double has four ropes each the same tension, each one quarter of the load. A triple has six ropes, all the same tension, each carrying one sixth of the load. So a 700 kg triple block has its load spread over six ropes each with a tension of 117 kg approx. when tensioned to breaking load (which is not at all recommended). While you are adjusting the system there is friction at each sheave which means the rope tension varies a little through the tackle, but with quality blocks friction is quite low, and within the margin of error in our knowledge of the load involved.
The flexibility of the mast is the main difference I would expect to see between the SO and SF series in the context of this thread, but I have not checked what mast each has. A bendy mast on a 3/4 rig will not require much backstay tension to make a significant adjustment, while a stiff mast and a masthead rig will require quite high tension to fully tension the forestay with a much smaller mast bend effect. A 7/8 rig would be somewhere between. However sailby2 and the many others you have spoken to clearly find the convenience of the tackle worthwhile and I would not ignore that experience, which always trumps simplified theory. I like his suggestion of a safety line in parallel with the smaller blocks in case of breakage.
rene460
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Post by Deleted on Nov 26, 2015 16:30:55 GMT
Thanks for the answers. I will purchase the triplex blocks with a load capability of 700kg. I will consider the securty line but I don't do racing with the boat. Just want to better trim the sails.
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Post by Deleted on Jan 22, 2016 17:19:37 GMT
Today the blocks were delivered. For the top I bought the Selden PBB70. It is 70mm just like the one used on that 36i. It seems it has a safe working load of 1500kg and a breaking load of 3000kg. So the breaking load is equal to the backstay wire. Now I wonder is the safe working load of 1500kg enough? The same is with th 50mm blocks, they have a safe working load of 700kg and a breaking load of 1400kg. Can anybody confirm this is okay.
The 70mm block comes with a standard shackle. Any idea how I connect this with the standard eye terminal of the backstay. It seems on the 36i the backstay terminal was replaced for a fork type.
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Post by davideso37 on Feb 19, 2016 9:51:56 GMT
Hi Marien,
We have the 2006 SO37 Legende and have had both the original mast head rig and the rig similar to the SF37 only the mast 300 mm taller. With the mast head rig the main purpose of the adjustable back stay is to stop the sag in forestay with the big headsail. The more it blows the more you pull on the backstay to stop the sag. After sailing you release the tension. With the fractional rig the backstay works to bend the mast and flatten the main sail. We use it all the time and adjust it many times during a race. As well as flattening the main it prevents forestay sag.
Regards
David SO37p Passion
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